When one wishes to attach plastic parts together, several processes exist to accomplish this. Among them are methods such as hot air bonding, sonic welding, vibration welding, adhesives, mechanical fasteners and infrared radiant heat.
One, in particular, infrared radiant heat has certain advantages over the other types. Generally, there is a desire to form a strong bond between the two parts quickly at a minimal cost. Additionally, it is preferable that the bonding process avoids touching the surfaces to be bonded themselves where they are to be bonded in order to assure uniform bonds from part to part and to reduce the cost of cleaning the bonding apparatus. Further, in many instances, it is preferable that there be no marring or distortion of the surface on the opposite side from that which is bonded, herein referred to as the viewing surface.
Infrared bonding in general can overcome many of these concerns. Infrared energy can be finitely pinpointed with a focal point or a mask to the exact area to be bonded in order to avoid overheating the plastic in adjacent areas that might cause unwanted distortion in the part. Infrared can heat the bonding surface to a high temperature, thus assuring a strong bond is formed between the two parts. The bonding surface can be heated very quickly with infrared and the timing and amount of heat application can be precisely controlled. Further, with infrared heating, there need be no contact between the heat source and the bonding surfaces of the parts, in order to minimize cleaning requirements for the tooling.
Nonetheless, there are some drawbacks, in general, to using infrared heat to bond plastic parts together. In the case of automotive interiors, for example, there are many parts made of plastic that are bonded to another part, but also need to have their viewing surfaces as distortion and mar free as possible since occupants of the vehicle will be able to see the surfaces. These surfaces are commonly referred to as "class A" surfaces. The need arises, then, to bond plastic parts having "class A" surfaces without burning or marring the surfaces. However, in the general application of focused infrared heat on the bonding surface in such applications, the "class A" surface cannot be prevented from being damaged. The finite focal point, while avoiding overheated adjacent areas is too intense and harms many plastic parts during the heating process. An inherent difficulty in working with infrared radiant heat to bond plastic arises, to heat the bonding surface sufficiently without causing warpage, burning or marring of the "class A" surface. A desire, then, arises to have non-contact focused radiant energy to heat the plastic at the bonding locations of each part, but not full intensity all at once to avoid problems associated with too much intensity. Preferably, this will be done while still minimizing, as much as possible, the time needed to heat the parts during the bonding process.
U.S. Pat. No. 5,151,149 to Swartz (hereafter '149 patent) discloses a method and apparatus for heating plastic for bonding using infrared, which attempts to heat the plastic fairly quickly up to its bonding temperature while avoiding burning the viewing surfaces. It discloses using infrared focused heat sources that move rapidly in repeated patterns to heat the plastic at a given point a little more on each pass.
Nonetheless, this process, as disclosed, has drawbacks in that shaking or rapidly moving the infrared heat sources while operating them causes a mechanical shock to the infrared heat sources and generally will reduce their average useful life. The bulbs typically have tungsten filaments, which, when hot, are prone to breakage from vibration. Start-up shock to the infrared bulbs can also reduce their useful life if started up each time they begin heating a new set of parts. Thus, it is desirable to avoid shaking the filaments of the infrared bulbs while heating in order to lengthen their lives, reducing the wasted time and expense associated with replacing heat sources.
Additionally, it is desirable that the infrared lamps remain fixed during the heat application. First, because it is desirable to keep the beam focused properly to assure the proper amount of heating. The '149 patent discloses moving the heat source throughout the heat application portion of the process, thus making it more difficult to maintain a properly focused beam. The '149 patent, then, discloses the need for a multi-axis robot to position and cycle the heat source to make sure it is focused properly, which is more costly than a simpler positioning mechanism such as a simple one-axis slow moving mechanism. Further, the process, as disclosed, lets the plastic cool slightly between each pass of the focused infrared ray across the particular point, which generally is not an optimum way to heat the plastic parts. It is advantageous to provide for uniform heating of the surfaces to be bonded by heating all of the surface area at the same time and only heating the exact areas needed. This affords maximum surface heat uniformity while creating a minimum of surface distortion. The '149 patent discloses continuously moving the heat source across the surface to be bonded, so it is impossible for this process to maintain the same material temperature across the bond surface at all bonding locations, and is unlikely that it can accurately and uniformly control the surface temperature at the instant of material bond, risking the creation of an unevenly bonded surface.
Accordingly, it is desirable to infrared bond plastic "class A" parts quickly without the drawbacks of prior infrared heat processes, such as discussed above.